The present invention generally relates to the preparation of semiconductor grade single crystal silicon, used in the manufacture of electronic components. More particularly, the present invention relates to a process for preparing a single crystal silicon ingot having a large diameter, in accordance with the Czochralski method, wherein the pull rate is varied during growth of a neck portion of the ingot in order to achieve dislocation-free growth over a reduced axial length.
Single crystal silicon, which is the starting material for most processes for the fabrication of semiconductor electronic components, is commonly prepared by the Czochralski (“Cz”) method. In this method, polycrystalline silicon (“polysilicon”) is charged to a crucible and melted, a seed crystal is brought into contact with the molten silicon and a single crystal is grown by slow extraction. As crystal growth is initiated, dislocations are generated in the crystal from the thermal shock of contacting the seed crystal with the melt. These dislocations are propagated throughout the growing crystal and multiplied unless they are eliminated in a neck region between the seed crystal and the main body of the crystal.
The conventional method of eliminating dislocations within a silicon single crystal (known as the Dash neck method) involves growing a neck having a small diameter (e.g. 2 to 4 mm) at a high crystal pull rate (e.g., as high as 6 mm/min.), to completely eliminate dislocations before initiating growth of the main body of crystal. Generally, dislocations can be eliminated in these small diameter necks after approximately 100 mm of neck is grown. Once the dislocations have been eliminated, the diameter of the crystal is enlarged, forming a crown or taper portion, until reaching the desired diameter of the cylindrical main body. The cylindrical main body of the crystal is then grown to have an approximately constant diameter by controlling the pull rate and the melt temperature while compensating for the decreasing melt level.
The neck, which is the weakest part of the silicon single crystal, can fracture during crystal growth, causing the body of crystal to drop into the crucible. Thus, conventional crystals having a Dash neck are typically grown to a weight of 100 kg or less to minimize stress on the neck. However, in recent years, progress in the semiconductor industry has created an ever-increasing demand for larger silicon wafers of a high quality. Particularly, more highly integrated semiconductor devices have resulted in increased chip areas and a demand for the production of silicon wafers having a diameter of 200 mm (8 inches) to 300 mm (12 inches) or more. This has resulted in the need for more effective neck growth processes which enable the elimination of dislocations and which prevent neck fractures, while supporting the growth of single crystal silicon ingots weighing up to 300 kg or more.
A general solution for preventing neck fractures in larger crystals is to increase the neck diameter. However, large diameter necks are generally undesirable, as they require larger seed crystals, which in turn produce a higher density of slip dislocations when contacted with the silicon melt. Thus, larger diameter neck portions require increased length, typically 150 mm or more, and thus additional process time, to effectively eliminate slip dislocations.
In order to minimize the generation of slip dislocations in a larger diameter Dash neck, Japanese laid-open application (Kokai) No. 4-104988 proposes a process using a seed crystal having a unique, conical shape at its apex. However, the unique seed crystal is complicated and expensive to process. Because the seed crystal is unique, a new seed crystal is needed for each crystal pull, regardless of whether dislocation-free growth was achieved. Thus, changing the seed crystal requires excessive process downtime, which adversely affects productivity. Furthermore, the process employs a heater embedded in the seed crystal holder. Having such a heater makes it more difficult to form a temperature gradient between the seed crystal and the neck portion, which requires the single crystal to be pulled at an extremely slow rate.
Another process for eliminating dislocations in a larger diameter Dash neck is disclosed in Japanese laid-open application (Kokai) No. 11-199384. Specifically, the application discloses a process whereby the length of the neck required to eliminate slip dislocations is shortened by repeatedly changing the neck diameter. The neck therefore has alternating sections of increased and decreased diameter, the reference describing the increased portion as having a diameter at least twice that of the decreased portion. However, while this process is said to provide a shorter length neck for growing large diameter silicon single crystals, the process is complicated and difficult to control because of the large difference in diameter between the increased and decreased portions, and because the target diameter of the neck must be constantly changed.
In view of the forgoing, it can be seen that a need continues to exist for a process that enables large diameter ingots of substantial weight to be grown by means of a neck having a comparably large diameter but short length.